1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to a CMOS (complementary metal oxide semiconductor) image sensor and a method for fabricating the same, to enhance the efficiency in condensing the light by forming a multi-layered micro lens with various materials having different refractive indexes.
2. Discussion of the Related Art
Generally, an image sensor is a semiconductor device for converting an optical image into an electric signal. The image sensor can be broadly categorized into a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS) image sensor.
The CMOS image sensor is provided with a photodiode PD for sensing the light, and a CMOS logic circuit for converting the sensed light to data of electric signals. At this time, as the amount of sensed light increases, photosensitivity of the image sensor improves.
In order to improve the photosensitivity, there is requirement for enhancing a fill factor of occupying a photodiode area in an entire area of the image sensor, or for condensing the light to the photodiode by changing the light-path of the light incident on the remaining areas except the photodiode.
For example, a micro-lens may be provided to condense the light. That is, a convex micro-lens is provided above the photodiode, wherein the convex micro-lens is formed of a material having great transmittance. As a result, the incident light is refracted by the convex micro-lens, whereby it is possible to provide more light to the photodiode. In this case, the light parallel to an optical axis of the micro-lens is refracted by the micro-lens, so that a focus is formed at a predetermined point of the optical axis.
Hereinafter, a related art CMOS image sensor and a method for forming a micro-lens therein will be described with reference to the accompanying drawings.
FIG. 1 is a schematic view of a related art micro-lens of a CMOS image sensor.
As shown in FIG. 1, a related art CMOS image sensor includes photodiode areas 11, an insulating interlayer 12, a protective layer 13, an R/G/B color filter layer 14, a top-coating layer 15, and a convex micro-lens 16. At this time, at least one photodiode area 11 is formed on a semiconductor substrate (not shown) to generate electric charges dependent on the amount of incident light. Also, the insulating interlayer 12 is formed on an entire surface of the semiconductor substrate (not shown) including the photodiode areas 11, and the protective layer 13 is formed on the insulating interlayer 12. Then, the R/G/B color filter layer 14 is formed on the protective layer 13 so as to transmit the light by respective wavelengths, and the top-coating layer 15 is formed on the R/G/B color filter layer 14. The convex micro-lens 16 having a predetermined curvature is provided on the top-coating layer 15, so as to condense the light passing through the R/G/B color filter layer 14 to the photodiode areas 11.
Although not shown, an optical shielding layer is provided in the insulating interlayer 12, wherein the optical shielding layer prevents the light from being incident to the remaining areas except the photodiode areas 11.
At this time, it is possible to provide a photogate instead of the photodiode.
Herein, the curvature and the height (‘A’ of FIG. 1) of the convex micro-lens 16 are determined in due consideration of the focus of the condensed light. Also, the micro-lens 16 is generally formed of polymer type resin, which is formed in a sequential process of deposition, patterning, and reflow. That is, the optimal size, curvature and thickness of the micro-lens 16 are determined in due consideration of position and shape of a unit pixel region, thickness of a photosensitive device, and height, position and size of the optical shielding layer.
On fabrication of the related art CMOS image sensor, the micro-lens 16 is a very important component for enhancing the light-condensing efficiency. The micro-lens 16 is provided to condense more light to the photodiode area 11 through the color filter layer 14 when the ambient light is irradiated.
The light incident on the image sensor is condensed by the micro-lens 16, and then is filtered through the color filter layer. After that, the filtered light is incident on the photodiode area 11 provided below the color filter layer 14. At this time, the optical shielding layer prevents the incident light from getting out of the path.
However, the micro-lens of the related art CMOS image sensor has the following disadvantages.
The light parallel to the optical axis of the micro-lens is refracted and transmitted to the photosensitive device being opposite to the micro-lens, thereby operating the device. However, the light being not parallel to the optical axis of the micro-lens is refracted and transmitted to the photosensitive device not to receive the light, whereby the device has misoperation.
Also, the amount of light transmitted to the photosensitive device is varied on the kind and thickness of the lower layer positioned below the micro-lens, whereby the light-condensing efficiency lowers, thereby deteriorating the picture quality.